Embedding passive components into circuit boards was developed as a way to decrease electronic size and improve performance. Circuit boards with embedded passive components optimize layout in all three dimensions, allowing more efficient use along the thickness of the board to decrease volume. Improved performance comes by way of shorter and more electromagnetic interference isolated connections between components. In addition to decreased size and increased performance, embedded passives decrease assembly steps required to connect components to the surface of the card and thus cost.
One customary practice for designing embedded passive components involves utilizing published embedded material properties. For example, sheet resistance may be given as 25 ohms per square (ohms/sq) while capacitance may be given as 1 nanofarad per square centimeter. These material specifications are commonly accompanied by upper and lower quality control limits, giving 25 ohms/sq a plus five percent and minus five percent variation, as an example. With this information, designs for embedded passives are set by defining the geometry such as the length and width of the material. Component length, width, and sheet resistance define resistance for an embedded resistor. That is to say, the resistance is the product of the sheet resistance and the ratio of length to width.
With lengths and widths defined, embedded component manufacturers are able to start the construction of a circuit board. Typically, a circuit board will have a structural dielectric base with a thin conductive copper foil attached. A circuit card with embedded components may also include a coating of electrodeposited metal such as nickel alloy on the conductive copper which would reside between the copper and base dielectric layers. Embedded components are formed as printed wires or traces by selectively etching the thin metal foil. For embedded components, that may include both metal layers of copper and nickel alloys. A multi-layer board is created by laminating this construction to other base dielectric layers with copper foil attached and bonded by way of heat and pressure and prepreg glue. Access of surface mounted components to embedded ones is by way of vias which are conductive barrels inserted into holes drilled through the various board layers. Vias attach conductive surface patches to embedded ones such as those from an embedded resistor.
The manufactured embedded passive board assembly is verified by measuring with a meter between vias exposed to the surface of the board and attached to embedded components. Utilizing the conventional manufacturing process described, the resistors in this example would not fall within the specified five percent control limits. This is primarily for two reasons; one, the way in which the material control limits are specified by the manufacturer; and two, the variations associated with the board manufacturing process.
In response to the two sources of undesirable process control, board manufacturers employ a corrective mechanical or laser trimming procedure accompanied by computer aided design geometry correction factors and quality control test points. Quality control test points are utilized as a means of further refining the manufacturers specified sheet resistance. Commonly specified values lack two items, statistical confidence levels on upper and lower control limits, and the minimum area in which sheet resistance is controlled within those limits with a particular confidence level. The smaller the area, the statistically less the likelihood that the sheet resistance will fall within control limits. To combat this, large embedded components, such as resistors, may be etched in handling areas of a laminate sheet. The resistors are large to offset local sheet resistance effects and variations associated with etching the geometry. The larger the resistor, the less probability a deviation in length to width ratio has on final resistance. These quality control resistors are measured, may be averaged, and compared to the manufacturers control limits. They provide a go/no go quality control checkpoint prior to etching any other embedded component. The problem associated with this checkout procedure still lies in the fact that the resistors are standardize to a set size to meet the manufacturers embedded material control limits restraining the size of the components.
Etching variations in the manufacturing process are tuned by way of computer aided design (CAD) or computer aided manufacturing (CAM) changes. Generally, board fabricators will make global changes to the received geometry component design to offset process variability in etching. Prior to etching, an etchant mask is placed down on the copper or nickel alloy material to protect areas where etching is not to occur. These areas may be increased to account for undesirable etching that occurs at the mask extents. Increases are made by way of CAD prior to placing the mask. These global changes, however, do not account for variations in etching that occur horizontally and vertically across the board. Like the embedded material, control points in the etching process are dictated by how they are averaged out over the entire surface area of the board.
Corrective trimming, by way of laser or mechanical means, is the third tool currently employed to reduce process and material variations issues. Prior to bonding etched layers of the circuit board together thus encapsulating components, embedded passive geometry is adjusted by removing resistive material such as nickel alloy. This allows for unidirectional adjustment in resistance thus limiting corrections to only increasing resistance. Making use of trimming requires CAD adjustments in component geometry to account not only for the unidirectional limitations of trimming process, but also for changes that occur when the board is exposed to heat in pressure during the lamination process.
A conventional embedded passive circuit card process will be described first to serve as a building block of understanding the invented procedure. This process is illustrated in FIGS. 1A through 1F. The manufacturer starts with a composite laminate, 100, made up of a dielectric layer, 101, a passive component layer, 102, and a conductive layer, 103, as shown in FIG. 1A. The dielectric layer is typically made up of a composite structure such as woven glass but can be any other material known in the art. The passive layer is typical comprised of resistive material, such as nickel alloy, for fabricating resistors or inductors, or a capacitive material for constructing capacitors.
The etching process starts with the application of a mask to a circuit board. The mask allows the outline of the desired design resistors, 104 shown in FIG. 1B, necessary quality control resistors, 105, to be formed by two etching steps. The CAD layout for the masking may be globally modified to offset for average etching variations across the board. The first etching step removes selective conductive layer material while the second removes selective passive layer material. The composite laminate is rinsed leaving the outlines of the resistors. A second mask is printed protecting everything from etchant except select portions of the quality control resistors. Copper is again etched away and the board is rinsed of its mask leaving the quality control passive components, labeled 106 in FIG. 1C, complete and ready for inspection. In the case of resistor components, resistance is measured and compared against passive material published control limits. If the inspect points are in compliance to the published data, a final masking and etching step is performed leaving the intended design passive component formed, 107 in FIG. 1D and ready to be tested.
At this point, the iterative trimming process starts. While this process may be automated, it is still very time consuming and costly. The design component, a resistor for example, is measured, 108 in FIG. 1E, to determine to resistance at this point of the process. Multiple measurements and trims, 109, are made until the resistor is within the desired range. This desired range may not be the ultimate end desired resistance as the component will be exposed to heat and pressure during the lamination process, FIG. 1F. Changes can be expected to occur and must be accounted for when the board is bonded with prepreg glue, 110. Vias, 111, are added to allow access to the embedded component from the top surface.
The current process describe in FIGS. 1A-1F employs CAD geometry corrections and quality control resistors in an attempt to minimize the effect material and process related variations have on the fabricated embedded component tolerance. The components, as for the case of embedded resistors, must be sized in the CAD in a manner that allows them to be lower in resistance, as the trimming procedure can only increase resistance. Trimming, performed prior to lamination process, must also account for the additional change the components will experience under heat and pressure during bonding.
Employing the current tools described above to tighten the control limits of embedded passive components offsets cost and performances advantages over their surface mounted counterparts. Laser trimming is an iterative procedure requiring multiple measuring and trimming steps to bring components within compliance. Trimming results in lower embedded component circuit yield and added cost. Trimming also decreases the likelihood the components will be successfully employed in high frequency electronic circuits. This is because trimming introduces variability in the current flow path and to the material itself. So while end component tolerance may be significantly decreased, cost is increased and applications are typically limited to low frequency applications.